Mary Lyon and Kathleen Marr have formed novel crystals of Photosystem II (PS II). PS II is a eukaryotic photosynthetic reaction center which converts solar energy to chemical energy in green plants. PS II is unique in its ability to evolve oxygen from water. Along with biochemical characterization, the structure of the complex was investigated by low dose electron microscopy and image analysis of negatively stained crystals. A projection map at 1.7 nm resolution has been produced. The unit cell was 11.5 by 16.1 nm and consisted of two monomeric units arranged around a central cavity. The monomeric units have four main density regions, which may correspond to the reaction center core and chlorophyll proteins that are linked to light harvesting complex (LHC II). The gap between the dimers is quite small, suggesting a functional connection between the two dimer halves (Lyon et al., 1993). Analysis of frozen hydrated specimens to produce a 2-D projection map has been completed. A collaboration has begun with David Mastronarde which has proven invaluable in the analysis of the frozen crystals. Specific image analysis programs which he has written, as well as modification of existing programs, have been used to solve some difficult problems in the analysis. Display programs written by Jim Kremer have also been employed. We are now using methods of antibody labeling to locate specific polypeptides in PS II. Statistical analysis of images of labeled vs control crystals is continuing, using programs written by David Mastronarde. Lastly, all digitizing has been done with HVEM equipment.